The present invention relates to methods and apparatus for pattern generation, and more particularly concerns improved methods and apparatus for rapid formation of patterns of high precision by a combination of mechanical and electrical drive.
In the manufacture of printed circuit boards, a circuit pattern is often produced as a high quality black-and-white image on photographic film or plate and transferred to a coated substrate. Features in such an image may have a positional accuracy of 0.001 inch over a distance of a much as 30 inches. The image on such film may be produced by a photoplotter comprising a light source, a lens system, a shutter, and a set of apertures for use in exposure of the film. A photohead of the photoplotter is moved relative to the film by high precision mechanical positioning devices in two axes and in two modes. In a "flash" mode, the shutter is opened and closed while the photohead is stationary to produce an image of a selected aperture at a particular position on the film, and then the photohead is moved to the next position, with closed shutter, where a second flash is caused. In a "draw" mode, the shutter is left open while the photohead is moved to draw a line of a width determined by the diameter of the selected aperture.
A major problem with such photoplotters is that they are very slow. If the image is of high density (more than 100 features per square inch) and large (several square feet), the image may take hours to produce. Many applications require several pieces of film for one job, and more jobs may require several days of continuous plotting for completion. A second problem is lack of versatility. The photohead can carry only a limited set of apertures of different sizes and shapes and is, therefore, unsuited for applications requiring a broad range of image features, such as phototypesetting. A third problem is the high degree of mechanical perfection required. In order to achieve desired positioning accuracy of features in the image, position of the photohead must be carefully maintained whenever the shutter is open. Maintenance of the position in the draw mode is even more difficult, because the photohead is in motion.
There have recently been developed precision photoplotters employing lasers to scan the film. The laser beam is rapidly turned on and off to produce dark and light areas as it scans, frequently in a rectangular raster. Since the image is produced in a raster mode rather than a vector mode, as is commonly used in photoplotters, the digital data must be converted into the raster format, a process that may take considerably longer than the plot itself. The amount of prior data processing required for this conversion varies with the square of the resolution of the raster. Precisely pointing the laser beam over a large area and modulating it on and off at exactly correct times may present problems as difficult as the mechanical problems of the traditional photoplotter.
Phototypesetters will generally expose a single line of print at a time, with the paper moving transversely to the line for exposure of successive lines. In a phototypesetter, one is concerned primarily with producing a high quality image, but geometric precision and precision positioning of the second images are of little concern and are not available.
Accordingly, it is an object of the present invention to provide a precision image-forming method and apparatus that avoids or minimizes above-mentioned problems.